1. Field of the Invention
The present invention relates to a color printing technique featuring multiple ink types.
2. Description of the Related Art
Color ink-jet printers are currently used on a wide scale as computer output devices. A common color ink-jet printer features a black (K) ink and multiple types of primary color inks, each having a cyan (C), magenta (M), or yellow (Y) hue. Any color of a color image can be reproduced by these multiple ink types. In particular, a gray color can be reproduced using either a black ink alone or all three types of primary color inks (CMY). The gray color reproduced using the three types of primary color inks (CMY) is commonly referred to as xe2x80x9ccomposite black.xe2x80x9d
So-called graininess (image irregularities) is used as an indicator for evaluating image quality. Image graininess is observed when ink dots form separately from each other. Consequently, image graininess is often a problem in comparatively bright image areas with a small number of ink dots.
FIGS. 1(A) and 1(B) illustrate in enlarged form a gray color area reproduced by a black ink alone, and a gray color area reproduced by composite black. The examples shown indicate that when a gray color is reproduced using composite black, the number of ink dots is three times greater than when black ink alone is used. Image graininess commonly increases with a reduction in the number of ink dots. Consequently, gray colors should preferably be reproduced using composite black as much as possible because of considerations related to image graininess.
However, shades other than gray sometimes appear to be present in a gray color reproduced using composite black. This phenomenon is primarily attributed to the spectral characteristics of illuminating light, as described below.
FIGS. 2(A)-2(D) illustrate the principle of color reproduction involving a gray color based on composite black. FIGS. 2(A) and 2(B) depict the spectral distribution PD50(xcex) of sunlight and the spectral distribution PA(xcex) of an incandescent lamp, respectively. FIG. 2(C) depicts the spectral reflectance RCB(xcex) of a gray color reproduced using composite black, and FIG. 2(D) depicts the color-matching functions x(xcex), y(xcex), and z(xcex) of a CIE XYZ color system. As is well known, the tristimulus values X, Y, and Z for expressing the colors of an image area can be obtained by a method in which the product of the spectral distribution P(xcex) of illuminating light, the spectral reflectance R(xcex) of the image area, and the color-matching functions x(xcex), y(xcex), and z(xcex) is integrated over a visible range of wavelengths xcex. In the present specification, the spectral distribution of standard light D50 (CIE) is used as the spectral distribution of sunlight. In addition, the spectral distribution of standard light A (CIE) is used as the spectral distribution of an incandescent lamp.
FIGS. 3(A)-3(D) illustrate the principle of color reproduction involving a gray color based on black ink alone. A comparison of FIGS. 2(C) and 3(C) indicates that the spectral reflectance RCB(xcex) of composite black is comparatively irregular and that the spectral reflectance RBK(xcex) of the black ink is comparatively smooth. The spectral reflectance of black ink and the spectral reflectance of composite black are noticeably different from each other but appear to have substantially the same color when viewed, for example, in sunlight (standard light D50). The phenomenon in which two colors with mutually different spectral characteristics appear to be substantially the same to the unaided eye under a light source is referred to as xe2x80x9cmetamerism.xe2x80x9d
When viewed in illuminating light other than sunlight (for example, under an incandescent lamp), a gray color printed with composite black sometimes appears to contain colors other than gray. This is because a composite black and a black ink have markedly different spectral reflectance R(xcex). Specifically, the tristimulus values (X, Y, Z)D50 obtained using the spectral distribution PD50(xcex) of sunlight and the tristimulus values (X, Y, Z)A obtained using the spectral distribution PA(xcex) of an incandescent lamp have comparatively large differences when composite black is involved. As a result, a gray color printed with composite black sometimes appears to contain shades or hues other than gray under an incandescent lamp. A black ink, on the other hand, has comparatively smooth spectral reflectance and comparatively similar tristimulus values (X, Y, Z)D50 and (X, Y, Z)DA. As a result, a gray color printed with black ink appears gray even when viewed under an incandescent lamp.
The dependence of the way in which a color is perceived by the unaided eye on the type of illuminating light (as in the case of composite black) is not a particularly desirable feature, creating a need for making a particular color look the same irrespective of the type of illuminating light. In other words, the appearance of a color should not depend on the type of light source used. Gray colors should preferably be reproduced by maximizing the use of black ink in order to reduce the dependence of color appearance on light sources.
In conventional practice, however, not much consideration is given to the dependence of color appearance on light sources during the reproduction of gray colors.
Accordingly an object of the present invention is to reduce dependence of color appearance of gray image areas on the light source used.
In order to attain at least part of the above and other related objects, there is provided a first method for printing images on a printing medium with the aid of a printing device capable of using multiple ink types. The printing method comprises the steps of: (a) preparing a black ink and multiple types of chromatic primary color inks, each of which has a cyan, magenta, or yellow hue; and (b) reproducing a gray color area having an arbitrary tone level using one or more types of ink selected from the black ink and the multiple types of chromatic primary color inks such that a color difference xcex94E of the gray color area in an L*a*b* color system is kept at about 4 or less when the gray color area is observed under two different light sources, such as standard light D50 and standard light A, respectively.
This printing method can print images with reduced dependence of gray color appearance on the light source because the gray colors are reproduced at color difference xcex94E of about 4 or less.
A second printing method of the present invention comprises comprising the steps of: (a) preparing a color conversion lookup table for converting first color image data expressed by a first color system to second color image data expressed by a second color system for the ink set; (b) converting the first color image data to the second color image data using the color conversion lookup table; (c) producing print data representing formation of ink dots at each pixel based on the second color image data; and (d) printing images in accordance with the print data. The color conversion lookup table has gray reproduction characteristics that allow a gray color area having an arbitrary tone level to be reproduced using one or more types of ink selected from the black ink and the multiple types of chromatic primary color inks such that a color difference xcex94E of the gray color area in the L*a*b* color system is kept at about 4 or less when the gray color area is observed under two different light sources, such as standard light D50 and standard light A, respectively.
In a preferred embodiment, one type of gray color is selected from a first type of gray color obtained using the multiple types of chromatic primary color inks but not using the black ink, and a second type of gray color obtained using the black ink and the multiple types of chromatic primary color inks; and the gray color area is reproduced using the selected type of gray color.
This approach is not limited to reproducing gray colors with black ink alone and allows gray colors to be reproduced by a so-called composite black or a combination of a black ink and composite black. It is therefore possible to reproduce a gray color area in which balance is maintained between image graininess and the dependence of color appearance on the light source.
The multiple types of chromatic primary color inks may include a dark ink and a light ink with respect to cyan and magenta; and the gray color area may be reproduced using the light ink but not using the dark ink for the cyan and magenta when a lightness level of the gray color area falls within a range of about 70/255-255/255.
According to this approach, image graininess can be improved because using light inks rather than dark inks increases the number of ink dots.
The present invention is also directed to a method of preparing a color conversion lookup table for converting first color image data expressed by a first color system to second color image data expressed by a second color system for an ink set composed of a black ink and multiple types of chromatic primary color inks, each having a cyan, magenta, or yellow hue. The method comprises the steps of: (a) preparing a plurality of gray color patches having a plurality of gray tone levels, respectively, the plurality of gray color patches including a first type of gray color patch printed using the multiple types of chromatic primary color inks but not using the black ink, and a second type of gray color patch printed using the black ink and the multiple types of chromatic primary color inks; (b) selecting one of the plurality of gray color patches with respect to each gray tone level such that a color difference xcex94E in the L*a*b* color system is kept at about 4 or less when the selected gray color patch is observed under different light sources, such as standard light D50 and standard light A, respectively; and (c) defining input and output of the color conversion lookup table to reproduce the gray color patch selected at each gray tone level.
In another aspect of the present invention, a gray color area is reproduced using both the black ink and the multiple types of chromatic primary color inks when the gray color area has a lightness level of about 150/255 or less.
This approach makes it possible to reproduce a comparatively dark gray color area in which balance is maintained between image graininess and the dependence of color appearance on the light source because this gray color area is reproduced by the combined use of a black ink and so-called composite black.
According to still another aspect of the present invention, a printing method comprises the steps of: (a) preparing a plurality of color conversion lookup tables for converting first color image data expressed by a first color system to second color image data expressed by a second color system for the ink set, the plurality of color conversion lookup tables including a first color conversion lookup table with comparatively desirable characteristics of dependence of color appearance on a light source and a second color conversion lookup table with comparatively desirable characteristics of graininess; (b) selecting one color conversion lookup table from the plurality of color conversion lookup tables and converting the first color image data to the second color image data; (c) producing print data representing formation of ink dots at each pixel based on the second color image data; and (d) printing images in accordance with the print data.
According to this method, images can be printed by selectively using a first color conversion lookup table with a comparatively good non-dependence of color appearances on the light source, and a second color conversion lookup table with comparatively good image graininess, making it possible to obtain an image quality that is in line with user preferences or particular image applications.
The present invention can be embodied in a variety of ways, specific examples of which include a printing method and device, a color conversion method and device, a method and device for creating color conversion lookup tables, a computer program for performing the functions of this method or device, a storage medium that stores this computer program and can be read by a computer, and a data signal embodied in a carrier wave containing this computer program.
These and other objects, features, aspects, and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiments with the accompanying drawings.